Thank you so much for posting your question in the Xfinity Forum. At this time, the Xfinity Stream app is only compatible with the following Amazon devices:
I would recommend either coming back to the forum later for updates to this or see here: https://www.xfinity.com/support/xfinity-apps/xfinity-tv-app-requirements/#Amazon
How is it you support ALL these other Amazon products but not the Fire TV Stick? You make it easier every day to cancel your services.